The Scottish champions were already 5-0 down when Armstrong stepped off the subs’ bench and he could do little to stem the tide which saw Barcelona add two more goals without reply.
But as Celtic prepare to welcome the Spanish champions to Glasgow for Wednesday night’s return fixture, Armstrong believes both he and his team-mates have made significant strides forward since that chastening evening back in September.
From the midfielder’s own perspective, he is now a central figure for Brendan Rodgers, pictured, and can confidently expect to be in the starting line-up this time around.
The 24-year-old is optimistic Celtic can continue an improvement in performance levels which has seen them claim creditable draws at home to Manchester City and away to Borussia Monchengladbach, albeit either side of a home defeat to the Germans, in Group C since that club record European defeat in Barcelona.
“This is what it is all about,” said Armstrong. “Everyone wants to play in the Champions League against the best players in the world.
“We have a chance to redeem ourselves on Wednesday after the disappointing away game. We’re looking forward to it.
“We can’t forget that game in the Nou Camp entirely. We don’t want to dwell on it either, but it’s important we learn from it and make sure we don’t make the same mistakes again.
“We don’t want a repeat of that game. It’s Barcelona, so they are bound to have a few surprises up their sleeves. We’ll hopefully deal with them as they come.
“That first game didn’t set a trend for the rest of them in the group.
“We’ve had good performances against Manchester City and Monchengladbach since then. We showed we learned from the game.
“Let’s not forget Barcelona are a top quality team. They are one of the best, if not the best, teams in the world. But in saying that, the way we lost was so disappointing.
“We’ve improved since then and hopefully we bring that into Wednesday night.
“It’s the Champions League and all the games are high pressure.
“There’s so much up for grabs on Wednesday night. Of course it’s a big ask. But if we want to be in Europe after Christmas, then we really need to take something from the game.”
It is an occasion where heroes can emerge, as was the case four years ago when Tony Watt entered Celtic folklore when he scored the late winner at Parkhead when Barcelona were beaten 2-1.
But Armstrong, who scored his third goal of the season as Celtic defeated Kilmarnock 1-0 at Rugby Park on Friday night to tighten their grip at the top of the Premiership, is not concerned by the prospect of personal glory.
“I don’t care who it is who becomes the new Tony Watt on Wednesday – as long as it’s someone,” he smiled.
“We have got goals in the team. That’s a big thing and when you have a squad you need to rely on everyone. There’s rotation that comes with that but we have guys like Tom Rogic, Patrick Roberts, Scott Sinclair, who can all chip in with goals, which is what we need.
“I have wanted to add goals to my game here. I was averaging ten a season when I was at Dundee United, so it was disappointing for me to only get four in my first full season with Celtic last year.
“I’ve got three so far this time, but hopefully I can keep building on it and getting into positions where I can add to my tally.
“I’m more comfortable now that I am operating in the middle of the pitch, rather than wide on the left where I was last season. I feel I can get into better positions and move about the team and into goal scoring positions more naturally. That’s what’s been pleasing for me.”
Armstrong’s display on Friday night further pressed his claims for international recognition with Scotland. Although he has been in a couple of squads under Gordon Strachan, he has yet to win his first cap, with his club boss Rodgers insisting last week it can only be a matter of time before he forces his way into the national side.
“The manager’s comments about Scotland are very flattering but I just need to keep working hard for Celtic,” added Armstrong.
“I need to focus on what I am doing here. It’s a dream come true to play in the Champions League, so that is my focus at the moment.”
The win at Kilmarnock set Celtic up nicely for a momentous week which sees the visit of Barcelona followed by Sunday’s Betfred Cup final against Aberdeen at Hampden.
“We were very good in the first half on Friday,” reflected Armstrong. “We used the ball very well and created a few openings. We should have been more than one ahead.
“In the second half it was a bit ugly. We didn’t control the game as we would have liked. It was a hard battle but we worked hard and got the three points. We want to produce the sort of performances we know we can week in, week out, but sometimes it is going to be like that.
“It was cold, it was on astroturf and the team was just back together after boys had been away on international duty. Three points was the objective and we managed to do that.
“It sets us up for this week. We have an important week ahead of us with the Champions League and then the cup final. We wanted to go in with some momentum and we’ve got that.”